Metal plating by chemical reduction with amineboranes



United States Patent 3,431,120 METAL PLATING BY CHEMICAL REDUCTION WITH AMINEBORANES Lavern M. Weisenberger, Highland Park, Mich., assignor to Allied Research Products Incorporated, Baltimore, Md., a corporation of Maryland N0 Drawing. Filed June 7, 1966, Ser. No. 555,715 U.S. Cl. 1061 11 Claims Int. Cl. C23c 3/04 ABSTRACT OF THE DISCLOSURE An aqueous chemical plating solution comprising a soluble nickel, cobalt, or copper metal salt, an amine borane and a soluble salt of heptagluconic acid.

This invention relates to electroless plating of metals and more particularly to an improved bath and process of chemical deposition of metals upon other metals, glass, ceramics and plastics by the reducing action of an amine borane in an aqueous solution of a salt of the plating metal containing a heptagluconate.

Heretofore electroless metal plating processes and baths have utilized as a reducing agent a hypophosphite such as sodium hypophosphite or potassium hypophosphite in an aqueous solution of a water soluble metal salt, such as a nickel salt together with various additives. One of the principal difficulties in such prior chemical reduction plating operations has been the accumulation in the plating solution of undesirable by-products, for instance, alkali metal chlorides, soluble phosphites and an acid, the particular acid depending on the source of metal employed. The unstability of such plating solutions necessitated its disposal after relatively short periods of use due to the accumulation of such by-products therein and their disadvantageous effects on the plating speed as well as the resulting plate.

Other prior art methods and baths have employed boron containing compositions as a reducing agent such as..amine boranes and boronhydride. In order to achieve stability of the plating baths, additives such as a watersoluble glycolate have been included in the bath composition.

It is therefore a principal object of the present invention to provide an improved bath for electroless plating of metals exhibiting improved stability characteristics.

Another object of the instant invention is to provide an improved process for electroless plating a metal upon a catalytic metal surface or a noncatalytic surface such as glass, ceramic, hard rubber, wood or plastic from a bath comprising an aqueous solution of the plating metal, said bath exhibiting improved stability characteristics.

It is yet another object of the instant invention to increase the operating life of a bath utilized for electroless metal plating.

A further object of the instant invention is to provide an electroless metal plating bath which deposits a smooth plate throughout the life of the bath upon a catalytic metal surface or a non-catalytic surface such as glass, ceramic or plastic surface.

Other objects of the invention will become apparent from the following detailed description of the invention.

The above objects may be accomplished by providing a new plating solution comprising an aqueous solution of 3,43 1,120 Patented Mar. 4, 1969 a metal salt, an amine borane and a soluble salt of heptagluconic acid. The metal can be chemically deposited from such plating solution by placing into contact with the solution such objects the surface of which is composed of iron, steel, nickel, gold, cobalt, aluminum, palladium, zinc, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, tin, silver, carbon and graphite. These materials function catalytically to cause a reduction of the metal ions by the amine borane present and thereby deposit such metals in a uniform layer on such surfaces.

Non-metallic surfaces such as wood, hard rubber, glass, ceramic or plastic surfaces can be treated prior to the plating operation to provide a surface Which is receptive to the deposited metal. Such non-metallic surfaces can be treated, for instance, by suitably sensitizing them according to a variety of techniques known to those skilled in the art. For instance, the non-metallic surface can be contacted with a dilute solution of SnCl and then rinsed. The SnCl -treated surface can then be contacted with a dilute solution of palladium chloride and thereafter contacted with the novel plating bath of this invention.

Additionally, non-catalytic surfaces can be activated by vapor deposition thereon of an active metal such as aluminum, iron, nickel, cobalt, palladium, etc., or by freshening and contaminating the surface with traces of a catalytic metal so that the catalytic particles are securely anchored in place upon the freshened surface. This can :be accomplished by dusting the surface with a finely divided catalytic powder. Moreover, the catalytic particles can be introduced into the non-catalytic material during a forming, molding or casting procedure. When the material to be plated according to this invention is, for instance, a plastic, the catalytic material in a finely divided state can be compounded with the plastic ingredients and thereafter cured. The surface can thereafter be freshened to expose the catalytic material by Wet or dry blasting, buifing, sanding, wire wheel brushing and the like.

Thus, according to the instant invention, the surface to be plated is catalytic either if it is composed of the aforementioned catalytic metals or it is sensitized or activated by the application thereon of a layer or film of such catalytic materials.

The novel plating bath of this invention comprises an aqueous solution of a metal salt, an amine borane as a reducing agent and a soluble salt of heptagluconic acid in amounts sufiicient to stabilize the plating bath. Conveniently, the metal to be plated can be nickel, cobalt and copper. The metal can be employed in the novel bath as a salt thereof, such as a metal sulfate, sulfamate and chloride, as well as the metal salt of mono and dicarboxylic acids such as an acetate, formate, citrate, tartrate, etc. Conveniently, the metal salt is present in the bath in amounts sufiicient to provide a concentration of metal in amounts ranging from 0.5 to 60 grams/liter, preferably 4.0 to 10.0 grams/ liter of the bath solution.

The amine borane employed as the reducing agent in the novel plating bath of this invention can be a primary, secondary or tertiary amine borane. Representative alkylamine boranes include, for instance, methylamine borane, dimethylamine borane, trimethylamine borane, ethylamine borane, diethylamine borane, triethylamine borane tripropylamine borane, pyridine borane, lutidine borane, morpholine borane as well as diborane diammoniate and ammonia borane (NH :BH The concentration of the amine borane in the plating bath can range between 0.05 and 20 grams/liter, preferably 1 to 10 grams/liter of bath solution.

The heptagluconate can be added to the plating bath as heptagluconic acid or as a water-soluble salt such as ammonium heptagluco-nate or a water-soluble alkali metal or alkaline earth metal salt of heptagluconic acid. Preferably sodium or potassium heptagluconate is employed. However, since the heptagluconate anion is, principally, the active ingredient conferring stability to the plating bath, the actual form in which the heptagluconate is added is immaterial as long as a material antagonistic to the bath is not present in the heptagluconate composition. The concentration of the heptagluconate can range between 1 to 160 grams/liter, preferably 10 to 100 grams/liter of bath solution.

The pH of the plating bath is maintained between 3.5 to 7, preferably between 6.0 and 7.0, conveniently by the addition of a buffering agent comprising a weak acid such as acetic, propionic, boric or the like. Generally the buffering agent will be present in amounts ranging from to 30, preferably 0 to grams/ liter of solution.

The plating operation conveniently can be carried out at ambient pressure and at temperatures ranging from to 65 C., and preferably, not greater than the boiling point of water. Preferably, the bath temperature ranges from 35 to 55 C.

The following are illustrative examples of particular processes and bath compositions in accordance with the present invention.

EXAMPLE I A nickel plating bath was prepared by dissolving 34.2 grams of nickel sulfate, 17.4 grams of potassium heptagluconate and 7.5 grams of dimethylamine borane in sufficient water to give 1 liter of solution. The pH was adjusted to 6.0-7.0 by addition of boric acid. The bath was maintained at a temperature ranging between 49-55" C.

A steel object was placed in the bath and a smooth uniform layer of nickel plated on it.

EXAMPLE II A nickel plating bath was prepared by dissolving 42.5 grams of nickel sulfonate tetrahydrate, 17.4 grams of potassium heptagluconate and 7.5 grams dimethylamine borane in sufficient water to give 1 liter of solution. The pH and operating temperature of the bath was essentially the same as that of Example I. A steel object was placed in the bath and a smooth, uniform layer of nickel plated on it.

EXAMPLE III A plating bath was prepared containing 36.5 grams of cobalt sulfate, 17.4 grams of potassium heptagluconate and 7.5 grams of dimethylamine borane with sufiicient water to make 1 liter of plating bath solution. Acetic acid was added to maintain the pH of the bath between 6.0 and 7.0 and the temperature was maintained between 49 C. and 57 C. A steel object was placed in the bath and a smooth, uniform layer of cobalt plated on it.

EXAMPLE IV A plating bath was prepared containing 30.0 grams of copper sulfate, 16.1 grams of potassium heptagluconate and 7.5 grams of dimethylamine borane. The pH of the bath was maintained between 6.0 and 7.0 by addition thereto of boric acid while the temperature was maintained at about 24 C. A steel object was placed in the bath and a smooth, uniform layer of copper plated on it.

EXAMPLE V The plating baths of Examples I-IV were prepared without employing potassium heptagluconate. These baths were operated under the same conditions and it was noted that metallic precipitates developed therein as opposed to the absence of any metallic precipitate in the baths of Examples I-IV even when used over prolonged periods.

Further, the baths of Examples I-IV were employed to plate a metal selected from the group of nickel, cobalt and copper on such metal substrates as iron, nickel, cobalt, aluminum, zinc, copper, brass, manganese, chromium, tin, silver and carbon. Additionally, the aforementioned plating metals were also deposited on various ceramic, glas and plastic surfaces which were pretreated as described above to provide a surface receptive to the plating metal.

It will also be apparent that prior to electroless metal plating, especially on a metal surface, the metal surface should be free of contaminating or plating interfering materials. Conveniently, such metal surfaces are cleaned by any of the conventional cleaning procedures such as mechanical cleaning, degreasing and acid pickling.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and modifications therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. An aqueous chemical plating solution comprising a soluble salt of a metal selected from the group consisting of nickel, cobalt and copper, a compound selected from the group consisting of primary, secondary and tertiary amine boranes and a soluble salt of heptagluconic acid, said solution having a pH of 3.5-7.0, said soluble metal salt being present in amounts sufiicient to provide a concentration of metal in amounts ranging from 0.5 to grams/liter of solution, said amine borane being present in amounts ranging from 0.05 to 20 grams/liter of solution and said soluble salt of heptagluconic acid being present in amounts ranging from 1 to 160 grams/ liter of solution.

2. The plating solution of claim 1 wherein the soluble salt of heptagluconic acid is selected from the group consisting of alkali, alkaline earth and ammonium heptagluconates.

3. The plating solution of claim 1 wherein the amine borane is an alkylamine borane selected from the group consisting of mono-, diand tri-alkylamine boranes wherein the alkyl moiety has l-3 carbon atoms.

4. The plating solution of claim 3 wherein the alkylamine borane is dimethylamine borane.

5. The plating bath of claim 1 including a weak acid buffering agent in amounts sufiicient to maintain said plating solution at a pH of 3.5-7.0.

6. A method of plating a metal on a catalytic metal surface which comprises contacting the surface with a chemical plating solution comprising a soluble salt of a metal selected from the group consisting of nickel, cobalt and copper, a compound selected from the group consisting of primary, secondary and tertiary amine boranes and a soluble salt of heptagluconic acid, said soluble metal salt being present in amounts sufiicient to provide a concentration of metal in amounts ranging from 0.5 to 60 grams/liter of solution, said amine borane being present in amounts ranging from 0.05 to 20 grams/liter of solution and said soluble salt of heptagluconic acid being present in amounts ranging from 1 to 160 grams/liter of solution and maintaining the pH of the solution between 3.5 and 7.0.

7. The method of claim 6 wherein the temperature of the bath is maintained at 15 to C.

8. The method of claim 6 wherein the soluble salt of heptagluconic acid is selected from the group consisting of alkali, alkaline earth and ammonium heptagluconates.

9. The method of claim 6 wherein the amine borane is dimethylamine borane.

10. The method of claim 6 wherein the pH of the 5 bath is maintained at 3.5 to 7 by adding a weak acid buffering agent.

11. The method of claim 10 wherein the buifering agent is boric acid.

References Cited UNITED STATES PATENTS 2,990,296 6/1961 Hoke 106-1 XR 3,063,850 11/1962 Mikulski 1061 3,211,578 10/1965 Gutzeit 1061 XR 6 3,259,559 7/1966 Schneble et a1 1061 XR 3,310,430 3/1967 Schneble et a1 1061 XR 3,338,726 8/1967 Berzins 106-1 XR JULIUS FROME, Primary Examiner.

L. HAYES, Assistant Examiner.

U.S. Cl. X.R. 

